This invention relates to hydraulic drive systems for powering the elevator of self-loading scrapers.
Such self-loading scrapers are used to load a wide variety of materials which have different loading characteristics affecting the speed and power requirements of the elevator. For example, some materials, such as sand or other loose materials, are easy to load due to their relatively low loading resistance and are best loaded at relatively high elevator speeds. Extremely strong undisturbed soils and the like are much more difficult to load and place a much greater loading resistance on the elevator. Frequently, such loading resistance is sufficiently great to cause the elevator to stall. Such stalling is highly undesirable because the elevator can not normally be restarted in the middle of a cut due to the loading resistance and the high inertia forces which must be overcome. This, of course, results in a large amount of lost productivity due to the time wasted in transporting and discharging of partial loads.
As it is also more difficult to move the scraper's cutting edge through such extremely cohesive soils, more of the available power of the scraper's engine must be applied to the tractor wheels in order to maintain the forward movement of the vehicle. Consequently, it is undesirable to draw the increase in torque needed to keep the elevator moving from the engine so that the engine is not unduly overloaded and forced into a lugging condition.
Another problem is that severe loading conditions frequently occur quite abruptly in operation due to changes in soil consistency and sudden changes in the depth of cut being made by the scraper due to varying terrain conditions. Consequently, unless immediate adjustments are made, the elevator will stall.